1. Field of the Invention
The present invention relates to a piston ring used in an internal combustion engine, and more particularly, to a stainless steel piston ring coated with a film having a superior wear-resistance, a superior anti-seizure property, and a quick break-in time (a superior initial adaptability).
2. Description of the Related Art
A hard chromium plating process has been widely adopted for a surface treatment of an outer sliding surface of a piston ring, because a piston ring coated with such a hard chromium plating film has a superior wear-resistance and causes less wear of a piston cylinder in contact with the piston ring. Recently, however, the power and rotation speed of an engine have been further increased, and thus the piston ring operates under more severe conditions. The hard chromium plating film can not sufficiently withstand such severe conditions, and accordingly, there is a need for an improved piston ring having a stronger wear-resistance and anti-seizure property.
Such an improved piston ring is plasma spray-coated with an Fe-Cr system film, is a stainless steel piston ring processed with a nitriding treatment, or is a piston ring plated with a composite film containing a ceramic powder, and these kinds of piston rings have been developed and adopted in some engines. The thus-developed piston rings have a remarkably improved wear-resistance and anti-seizure (anti-scoring) property, when compared with the conventional piston rings, but also have the following disadvantages.
The piston ring coated with a Fe-Cr system plasma sprayed film has a superior wear-resistance and anti-seizure property, but causes a large amount of wear of the piston cylinder. Furthermore, when this piston ring is exposed to a corrosive atmosphere, the metal-sprayed film is often separated from the ring. The piston ring coated with a composite plating film containing a ceramic powder has a superior anti-seizure property, but the matrix strength of the coated composite film is a problem in that the film cracks and is separated from the piston ring during an engine operation. The stainless steel piston ring processed with a nitriding treatment has a superior wear-resistance and causes less wear of the cylinder, but has a low anti-seizure property and causes problems in an engine under a high heat-load.
To improve the nitrided stainless steel piston ring, the outer sliding surface of the piston ring (i.e., the nitrided layer) is coated with a titanium nitride film having a desired (predetermined) thickness, by an ion-plating process. The current titanium nitride film however, does not seat in a cylinder wall quickly, and if the film has a surface roughness of more than 0.6 .mu.m Rz, it causes a large amount of wear of the cylinder.